Chlorophyll is a green pigment found in plant cells, algae cells, and cyanobacteria. The primary function of chlorophyll molecules is radiation absorption, which provides energy for photosynthesis. Measuring chlorophyll in plants (e.g., primarily leaves of plants) may be helpful in determining if the plant has proper conditions (e.g., habitat, nutrients, lighting, etc.) for optimum growth. For example, when unfavorable growth conditions result in plant physiological stress, leaf chlorophyll content often begins to decrease. Thus, chlorophyll measurements may be useful in the detection of plant physiological stresses, which may be a result of drought, chemicals (e.g., herbicides), biological influences, or other influences. As a result, early detection of such reduction of chlorophyll may provide an opportunity to identify and reverse the physiological stresses on the plant.
Leaf chlorophyll concentration is generally most accurately measured by extraction of chlorophyll in a solvent followed by in vitro measurements in a spectrophotometer. However, non-destructive, in situ, optical techniques have become widely used to provide a relative indication of leaf chlorophyll concentration. Some optical techniques include monitoring the reflectance of light incident on a plant sample, such as with U.S. Pat. No. 6,020,587 (Spiering et al.) that describes using two light sources with different wavelengths and detecting reflected light from a plant sample to obtain an estimate of chlorophyll content. Another optical technique includes monitoring the transmittance of light after passing through a plant sample, such as with U.S. Pat. No. 7,746,452 (Fuchigami et al.) that describes using light sources with different wavelengths and detecting light transmitted through the plant sample to obtain an estimate of that is representative of chlorophyll content.
Such handheld chlorophyll meters are widely used to estimate leaf chlorophyll concentration as an alternative to destructive sampling techniques, but non-uniform chlorophyll distribution causes the optical measurement to vary widely among species for the same chlorophyll concentration. Over 30 studies to date have sought to quantify the in situ/in vitro (optical/absolute) relationship, but neither chlorophyll extraction nor measurement techniques for in vitro analysis have been consistent among the different studies. In addition, the reported optical/absolute chlorophyll concentration relationship has varied widely, sometimes even within the same species of plants.
Two commercially available chlorophyll meters are currently widely used (e.g., Minolta, model SPAD-502; and Opti-Sciences, model CCM-200). Such commercially available chlorophyll meters output values related to chlorophyll content that are unique to the devices, but these meters do not output values that are in units of chlorophyll content. For example, the Minolta chlorophyll meter displays an output that is a “SPAD” value, while the Opti-Sciences meter displays an output as a “CCI” value. The SPAD values and CCI values are influenced by the chlorophyll content; however, they do not have a linear relationship with the absolute chlorophyll concentration. In other words, a SPAD value of 20 would not necessarily correspond to approximately twice the chlorophyll content as a SPAD value of 10. As a result, users may merely know that chlorophyll content is high or low based on one SPAD value being compared relative to another SPAD value; however, the user may not know the actual amount by which the chlorophyll content actually differs when comparing different SPAD values. CCI values have similar limitations. These SPAD values and CCI values are discussed in further detail below.